The present invention relates to a pumping system for multiple-phase fluids. More specifically, it relates to a multi-phase pumping system that includes multiple-phase pumps with mechanical differential units, which are able to pump liquids only, gases only, or liquids and gases simultaneously in any ratio, eliminating the recirculation of fluids. The system of this invention is particularly useful in the oil industry. The invention also refers to the method used by the system put forward here for pumping multi-phase fluids.
In industry, particularly the oil industry, there are many situations in which liquids and gases are found together or mixed together, and need to be supplied with power for transporting through pipelines.
There are two distinct types of conventional equipment used to do this: pumps and compressors.
Pumps work efficiently with liquid, though not when gas is present; when gas is present the pump may cease to function, depending on the percentage of gas.
The same behaviour is seen in reverse with compressors.
Thus, if for example energy is to be transfered into a multi-phase flow in order to facilitate long-distance transport, it becomes necessary to separate the constituents into liquid and gas flows. For this op ration one uses the liquid- and gas-phase separators. In this way, following separation, the liquid flow will be directed to a pump, there to be supplied with energy and transported, while the flow of gas will be directed to a compressor for the same reason.
Generally, to work with flows of fluids at high-pressure, the separators are heavy, bulky vessels which are fitted with control and safety systems in order to maintain the correct liquid level for operation. Besides being expensive they overload the production system, especially in applications where there are limitations on space, weight or the complexity of the components installed (for example, off-shore oil-production rigs and/or sea-bed oil-production systems).
In order to do away with use of separators, industry has set about using, adapting and developing mono-phase liquid pumps and mono-phase gas compressors which can function as multi-phase pumps, pumping in two phases, liquid and gas.
Many types of multi-phase pump are under development such as: piston pumps, diaphragm pumps, single and/or multiple screw Moineau, spiral-axial, or centrifugal pumps. However, until now, none of these designs has yet reached the stage of large-scale application in industry. Those that attained the most widespread application were multi-phase twin-screw pumps and the rotary-dynamic pumps of the spiral-axial type.
A basic problem of the multi-phase fluid pump is the Circulatory Flow (C.F) of fluids, to be explained in detail later herein.
An Oscillating Chamber String (OCS), disclosed by Sulzer Pumps (Germany) in 1989-1990, is a multi-phase pump with variable capacity of the pistons which solves the C.F. problem, though in a more complex way than that adopted by the present invention. The OCS piston pump has a positive displacement action. The pistons and connected sheaths connected in a set produce a multiple-stage pump. The travel of each piston is variable. A control-system and motors connected to each piston reset the piston""s travel, so as to maintain equal pressure increments in the component stages of this design.
A twin-screw pump is normally used to pump liquids, at which it gives good performance, and it has been adapted to serve as a multi-phase pump. This is also a positive-displacement pump, made up of two metal screws and two metal sheaths, producing cavities of equal volume, which move by suction to discharge the pump, in order to drive the fluids. The screws and sheaths form metal seals between the cavities; in other words, each cavity demarcates a stage of the pump.
The twin-screw pump displays the following disadvantages, brought about by the phenomenon of Circulatory Flow (C.F.):
1. durability is reduced through the increased proportion of gas;
2. energy-efficiency is reduced through the increased proportion of gas, possibly declining to zero;
3. it is unable to pump high proportions (for example, over 95%) of gas, or gas alone.
There follows a description of C.F. and its effects.
The mono-phase gas compressor has variable volumes at each stage, being unable to pump liquid, hence an Excessive Rise in Pressure (E.R.P.) would arise at each of its stages. In order that the liquid will be pumped while avoiding E.R.P, the twin-screw pump exhibits stages or cavities at constant volumes. Hence, there would be no reduction in the volume of gas entering the cavity, so pressure would not rise. Thus, suction pressure would be maintained in all pumping stages, and discharge pressure would increase only at the final stage, when the cavity communicates with the pump discharge. This is certainly not what actually happens, since the final stage does not resist any increase in the required pressure. If it did resist, there would be no need for an extra stage.
Single-stage pumps do not present this problem, since the single stage resists any increase in the pressure required.
Under the law of conservation of mass, the flow-mass must be constant at all stages of the twin-screw pump. Fluid pressure rises while the cavity is moving; in other words, pressure rises from one stage to another. With that rise in pressure, the volumetric flow of fluids declines, allowing a state of gas equilibration, and as a result it cannot succeed in completely filling the cavity. Thus, it is filled up with fluids which will not normally drain away. Those fluids that remain in place, occupying cavity spaces that pass through them, represent the C.F. of the fluids.
By way of illustration, let us suppose a twin-screw pump with several stages, compressing gas with a pressure value of 1 Absolute Unit of Pressure (AUP) of suction and 10 AUP of discharge. The volumetric suction gas-flow is at its maximum whereas, when discharged, this flow declines to {fraction (1/10)} of the volumetric gas flow in the first cavity. Consequently, {fraction (9/10)} of this volumetric flow will need to be supplemented by fluid originating in C.F: in other words, this {fraction (9/10)} of the fluid continues to occupy the cavity, moving to the previous cavity, as long as this continues to occupy the position of the last cavity.
This same phenomenon occurs with the remaining cavities; however, the C.F. will be lower, since it depends on the relationship between the cavity and pump suction.
The return, or greater C.F., occurs at the final cavity where there is greater pressure, and the smaller C.F. at the first where there is less pressure. However, linear distribution of pressure does not occur, because the return flow, being much greater in the higher stages, is impeded by the clearances found between the cavities. Therefore, in the presence of gas, the higher stages function with a greater Rise in Pressure, or greater E.R.P.
The twin-screw pumps are installed with a minimum clearance between the screws and sheaths, when they function as virtually non-compressible liquids. These pumps are not multi-phase and do not compress gas, because an E.R.P. would arise at the stages. In order to make them multi-phase, designers reduce the E.R.P. increasing the clearance between screws and sheaths so that the remaining stages will function.
Supposing a discharge of a liquid pump should be linked to its own suction by means of a choke or control valve, so that 90% of the pumped flow returns. If the hydraulic power of the pump were 10 Units of Power, 9 of those units would be dissipated at the choke in the form of heat. If the choke did not exchange heat with the environment (considering that this is an adiabatic process), the result is equivalent to installing a heater with the same 9 units of pump-suction power, in order to heat just 10% of the flow passing through the pump. However, the return of fluids at the twin-screw pumping stages causes overheating, similar to the overheating caused by the choke when working under an adiabatic rxc3xa9gime.
Fluids that return without leaving the interior of the twin-screw pump cannot cool it down, because each time they return, they are heated on passing through the hydraulic sealing areas of the cavities.
Experimental data on twin-screw pumps having more than one stage shows that the total power consumed by the pump does not depend on the volumetric gas ratio. This phenomenon occurs not only in single stage pumps, since the power declines greatly while the volumetric ratio of gas rises. C.F. accounts for this phenomenon.
When there is only liquid, hydraulic power is around at most 75% of the total energy consumed by the pump. Therefore it reduces the ratio of gas linearly, to zero (0%).
When there is only liquid, heat generated by the pump is caused by physical friction, in the order of 25% of total energy consumed by the pump. Therefore heat increases linearly with the ratio of gas, owing to C.F. of fluids and gas compression, until reaching a maximum value equal to the pump""s total power (100%) when there is only gas. In other words, the heat generated increases approximately fourfold, while cooling of the pump is greatly reduced, since the thermal capacity of gas is far lower than that of liquid.
When there is only gas and the compression ratio is 1 to 10, heat generated by gas compression, physical friction and C.F. is in the order, respectively, of 20%, 25% and 55% of total energy consumed by the pump. Its energy-efficiency is obtained by the compression effort, which is roughly equal to the heat generated by compression; in other words, energy-efficiency is in the region of 20%.
When the compression ratio is 1 to 100, heat generated by gas compression, by physical friction and by C.F. is in the order, respectively, of 3%, 25% and 72% of total energy consumed by the pump. Energy-efficiency is in the order of 3%. C.F. is responsible for the greater proportion of energy dissipated.
Apart from overheating and poor energy-efficiency, E.R.P. and C.F. cause, respectively, distortion and excessive decay of the pump""s screws and sheaths.
E.R.P. can be prevented by increasing the clearance between screws and sheaths, to facilitate C.F. However, overheating, low energy-efficiency and decay cannot be prevented with this type of pump. Differentiated gaps reduce the rise in pressure at some stages, but increase the rise in pressure at others. Larger clearances reduce pressure increases, but increase the number of pump stages. However, in neither case are these unwanted effects avoided altogether.
Multi-phase pumps made with high clearances to prevent E.R.P. also have the disadvantage of failing to work when there is C.F. of low-viscosity fluid, which produces little difference in pressure between stages. This happens mainly when there is a high proportion of gas.
Generally speaking, overheating restricts the operation of multiple stage pumps to gas levels below 90%. Above this level, the liquid portion is insufficient to cool the pump down. Nevertheless, E.R.P. (which causes distortion) and C.F. (which causes decay) restrict the application of the pump even more, to gas values below 20%, namely to values close to the permissible drainage when pumping only liquid.
These factors apply equally to other types of pumps with more than one stage, but not to single-stage pumps.
Consider a single stage twin-screw pump with suction pressure of 1 Absolute Unit of Pressure (AUP) and discharge pressure of 10 AUP. The cavity is filled up with fluids at 1 AUP while open for suction. As long as the screws continue rotating, the cavity communicates with the discharge, initially through a small opening. The fluids from the discharge will return into the cavity, compressing the gas until the 10 AUP pressure level is reached. In the process, little energy is dissipated in the form of friction, because fluids do not return, straining the seal. Friction is very slight, because the cavity opens so that fluids return without any difficulty. Energy is converted mainly from pressure into kinetic energy and vice-versa. Following this return, movement is started up and fluids will leave the cavity, as long as the cavity diminishes.
In the presence of gas, it turns out that single stage pumps display C.F, with no loss of energy-efficiency. Therefore, multi-phase pumps are more efficient in terms of energy and durability than a single stage.
Single stage pumps, or any other pump with more than one stage, installed in such a way that forced C.F. does not take place, will work at any gas ratio since there is no E.R.P. or decay, and also less heating.
An example of a multiple stage pump installed to avoid C.F, is the OCS positive displacement piston pump. With the aim of maintaining an equal increase in pressure at all stages of this pump, a complex system of measurements and pressure controls is necessary to activate the motors which reset the piston travel.
Nonetheless, OCS piston pumps display the following disadvantages:
1) there is E.R.P. and C.F. while the response time of the control-system is slow compared with changes in the proportion of gas, especially when there is intermittent drainage in the pipework supplying the pump;
2) there is poor reliability, due to the complexity of the control system;
3) there is higher energy consumption, due to the motors which change the piston travel.
E.R.P. and C.F. occur both in compressors with more than one stage at which they pump liquid, and in pumps with more than one stage (piston, diaphragm, single- and/or multiple-screw, Moineau, gear, spiral-axial, centrifugal, etc) when they compress gas. Resolving the problem of E.R.P. and C.F. in these pumps equates to solving the same problems in such compressors; in other words, the difficulties of converting a liqiuid pump into a multi-phase pump are the same as those involved in converting a gas compressor into a multi-phase pump, because E.R.P. or C.F. are inevitable in all these fluid machines.
Fluid machines are devices that supply (pump, compressor, ventilator, extractor-fan, ejector energy to fluids) or receive (water-wheel, Pelton turbine, Francis turbine, wind-tunnel) energy from fluids. These are also known as flow machines.
Nonetheless, despite all the new developments, these mono-phase pumps are not entirely suitable for multi-phase fluids, since they are not multi-phase pumps and do not apply multi-phase principles; in other words, they cannot be properly adapted to the variable compressibility of multi-phase fluids. Finally, they do not show variable volumetric flow at each stage.
Positive displacement pumps of the OCS type give better performance than that of mono-phase pumps, currently under development for use in multi-phase service, as they do not show the unwanted effects of E.R.P, nor of C.F.
However, when these are compared with pumps that work only with liquid and compressors working only with gas, existing multi-phase pumps display at least one of the drawbacks already mentioned in the present specification for twin-screw pumps.
Depending on operational requirements, these disadvantages greatly restrict the application of most existing multi-phase pumps. Even for small and medium quantities of gas, the possible occurrence of intermittent drainage (separate receptacles for gas and liquid) in the supply pipework can restrict the scope of application of these multi-phase pumps even more. The literature reveals various patents relating to pumps for multi-phase effluents.
U.S. Pat. No. 5,253,977 describes an axial pump which makes possible the pumping of a fluid with a dual liquid-gas phase at high flow-rates. It consists of a single-part rotor including a hollow shaft, inside which there is a pulsating contraction system (rotor and diffusor). This system is installed inside a unit comprising a stack of washers, inside which stretchers are fixed. Each stretcher is made of two half-stretchers, in such a way as to allow each stretcher to be installed in the rotor wheel. The whole is sealed by flanges at the edges, on which the rotor is mounted for rotating. The pulse system can also be manufactured on the external surface of the unit.
This US patent makes no claim to be a new method of multi-phase pumping, since it is concerned with a pump of the axial type, widely used in industry, especially for mono-phase pumping of liquid and gas. What it does claim to be is a new method of manufacturing this pump, so that the rotor shaft unit will be pre-balanced, minimizing vibration caused by this unit. However, vibration caused by the heterogenous mass of multi-phase fluid, at varying density phases, still occurs.
U.S. Pat. No. 6,135,723 describes a multiple-stage pump with a housing that defines multiple stages, each stage having an internal rotor-box, each box having an input and output for which there are no pumps. A rotor unit is contained inside the housing: under operational conditions, this housing extends right through all the stages. The rotor units and their boxes are made so as to give a volumetric entry supply rate at the final stage (downstream current or output) that is less than that of the first stage (upstream current or input). Multiple fluid channels connect the non-pumping chambers in order to allow the pump to drive the liquid in such a way that, as the rotor unit rotates, a current of fluid entering the pump input will be subjected to pumping action to move the flow of fluid to the output through the pump""s output.
This pump does not prevent circulatory flow and its attendant drawbacks: poor energy-efficiency, excessive rise in pressure (E.R.P.) and excessive decay. These problems are partly transferred outside the twin-screw pump.
In conventional twin-screw pumps, circulatory flow (C.F.) occurs between the rotors or screws and the pump sheath, damaging them in the process. In U.S. Pat. No. 6,135,723, part of C.F. occurs outside the pump itself.
The remaining C.F. occurs inside the pump, between the stages or screw passages between the areas without a screw, in the same way as happens in conventional twin-screw pumps. In order that no C.F. will occur, there cannot be more than one stage between the areas without a screw.
The fact that the pump""s discharge stages will be fewer than the suction stages prevents or reduces C.F. when more gas is coming in. However, C.F. remains greater when more liquid is entering.
Hence, this patent fails to resolve the main problems of C.F.; namely poor energy-efficiency and decay. It merely reduces one problem: the rise in pressure. The low energy-efficiency and decay still persist and are caused by C.F. in the pump""s external fixtures (pipes, valves, accumulators and auxiliary pumps).
On the other hand, the patent literature mentions differential-action units for automotive systems. These units are used in the motor industry to distribute the energy of a shaft from the engine to each axle connected to a wheel, in accordance with U.S. Pat. No. 3,886,813 and U.S. Pat. No. 4,577,721 among others.
U.S. Pat. No. 4,109,595 describes a multiple-differential used in the textile industry.
The differential-action unit is a simple device, with few gear wheels (normally four). In the case of a motor vehicle it allows the wheels to rotate at the same speed on the straight but at different speeds on bends; in other words, wheels on the inside of a bend rotate more slowly than those on the outside, i.e. while they are covering different distances. Both on the straight and on bends, torque is distributed equally to the wheels. The differential-action unit swiftly fulfils this function, accurately and automatically, and more efficiently than other systems. Hence it is used in most motor vehicles.
In accordance with the concept of this invention, a differential may, on being coupled to a pumping system, cause the volumetric fluid flow being pumped to change, thus reducing the fluid""s C.F.
U.S. Pat. No. 2,698,576 (Strub) describes a pumping system for liquids that makes use of mechanical differentials. The Strub system is used for pumping of high pressure liquids, around 3,000 atm. This solution has not been used in the industry because the mechanical differentials are unnecessary for use. This pumping is easily obtained without differentials, decreasing the volumetric capacity in each series stage of the pump, according to the liquid compressibility, as the pressure goes up. The dimensioning of this pump for liquid in high pressure, without mechanical differentials, is similar to the dimensioning of as compressors with more than one stage in series. In respect to the compressor, the low compressibility of the liquid results in lesser difference between the volumetric capacities of the pump stages, while the high compressibility of the gas results in greater differences between the volumetric capacities of the compressor stages.
Thus, technology relating to multi-phase pumps still needs to be further refined in terms of pumping efficiency, particularly with regard to fluid recirculation and C.F. aspects. Such refinements include the Multi-phase Pump with differential units, giving low or zero C.F, described and claimed in the present application.
The multi-phase pumping system according to the invention is defined in claim 1. It may include a housing enclosing the multi-phase pump unit, a differential unit and multiple stages, united by means of shafts, the first of these being the driving-shaft, which is rotated by a motor. This drive-shaft activates a differential unit, which in turn rotates the next two shafts which drive the first two pumps connected in a set, the differential units providing the necessary rotation compensation, so that each pump displays a variable volumetric flow, controlled by the compressibility of the fluid, such that the C.F. of that fluid is reduced or altogether eliminated.
By comparison with pumps that work only with liquids and compressors working only with gas, existing multi-phase pumps display at least one of the following drawbacks: great complexity; durability that is reduced with increases in the ratio of gas; and reduced energy-efficiency with increases in the ratio of gas, tending down to zero. They will not pump high proportions of gas, or gas alone.
The Multi-phase Pumping System in accordance with the invention reduces or eliminates C.F. This is achieved through the use of mechanical differential units, and has the aim of pumping only liquid, only gas, or liquid and gas simultaneously in any ratio, without producing any of the previously-mentioned drawbacks. The use of mechanical differential units provides a simple way of substantially reducing or totally eliminating C.F, which is the main source of these shortcomings.
Also, under the operating method deployed in this pumping system, a drive-shaft activates the differential unit which, in turn, rotates the shafts which activate pumps arranged in a set. In order to be pumped, multi-phase fluid, liquid or gas in any proportion enters through the suction pipe of a first pump where the pressure rises, it passes to the discharge pipe of that pump, and enters through the suction pipe of a second pump where there is a further rise in pressure, and finally leaves through the discharge pipe of the second pump.
When the fluid is liquid alone, both pumps at each stage rotate at the same rate, to produce equal volumetric flows. Therefore, the multi-phase pumping system according to this invention for reducing or eliminating C.F. works analogously to the wheels and differential unit of a vehicle running in a straight line.
As used above, the word xe2x80x9cstagexe2x80x9d has the following meaning. When pumps are arranged in a set, each pump represents one stage, since each causes an incremental step-change in pressure. Hence, the increments in pressure at each pump are cumulative, and the mass of fluid passing through the two pumps is identical.
Yet when the pumps are in parallel, the mass of fluid passing in each pump increases, while the pressure does not. In this situation there is just one stage, since there is hardly any increment in pressure set by the suction and discharge pressures, which are equal for all pumps.
When any proportion of gas enters, the volumetric flow of the second-stage pump remains less than that of the first-stage pump, because gas is compressible, and pressure at the first stage is lower than at the second, as the pumps are linked into a set. The pump-shaft of the first stage rotates more rapidly than that of the second, since the differential-action unit makes the necessary rotation compensation, in the same way as do the wheels of a vehicle rounding a bend.
The invention envisages further pumping systems with more than one differential-action unit, in order to produce more than two pump stages.
The invention also provides a multi phase pumping method as defined in claim 12.
Systems with one or more multiple-differentials are also envisaged for the invention.